A 61-year-old man suddenly became euphoric and talkative. Later the same day, he developed hemichoreic movements of the left limbs. The patient fulfilled the DSM-IV criteria for a manic episode by abnormally and persistently elevated mood, decreased need for sleep, high distractibility, pressured speech, increased goal-directed activity, and hypersexuality. The mood changes persisted for several weeks. Magnetic resonance imaging of the brain showed a right thalamic infarction. The co-occurrence of hemichorea and mania caused by focal thalamic lesion is very rare. It may be explained by dysfunction in basal ganglia thalamocortical circuitry.